(Much better to save money and buy a conversion bulb than it is to buy an entirely new lighting system, right?)
Can't say I would agree with you there. For about 2 thirds of the cost of that 250W conversion bulb I bought a 600 W HPS bulb, reflector & ballast as a system from ebay including delivery. My own PowerPlant 250W HPS system is rated for MH as well without requiring any switch & the source I use for bulbs is £11.99 for 250W. Due to the size of the standard horizontal relflector, depth of throat between the fitting & the metal etc it would not fit the diameter of that conversion bulb. Without much work I could find a few partial or complete grow systems based on HPS for which there is much evidence to suggest the ballast can accept a standard MH bulb without the need for a conversion version as well as those stated as being switchable & have change from the cost of the conversion bulb.

Sometimes it s cheaper to throw out awkward equipment & renew a system to something more scaleable & conforming to popular standards.

Each to their own though & good growing to all.